Carrier current apparatus



13% 7, 1943- E; w. KENEFAKE 4 2,336,253

7 CARRIER CURRENT APPARATUS Filed March '7, 1942 3 Sheets$heet 1 l9 :9czwisfir cz'a'a'ss-r APPARATUS APPARATUS m I7 I CARRIER 5 CURRENT mAPPARATUS 2 CARRIER CURRENT n 7 APPARATUS 2 23 I4 f CARRIER CURRENTAPPARATUS 1/ r5. AWL 1 Hi ttorney v Filed March 7, 1942 3 Sheets-Sheet 2Fig.5.

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mwmuuwmuuauwmmmw V L m CABLE LENGTH- FEET Inventor: Edwin W. Kenefake,

His ttorney Patented Dec. 7, i943 CARRIER CUR RENT API'ARATUS Edwin W.Kenefake, Schenectady, N. Y., assignor to General Electric New YorkCompany, a corporation of Application March 7, 1942, Serial No. 433,824

Claims; (01. 177-352) My invention relates to carrier current appa- 1ratus, and more particularly to means for coupresents a purely resistiveimpedance to the car- I rier current apparatus. Such anarrangement issatisfactory from an operating standpoint where the carrier currentapparatus is in close proximity to the power transmission line sincetuning means for the coupling capacitor and transmission line may thenbe in the same enclosure as the carrier current apparatus and is thusreadily accessible for easy adjustment without special precautions toprotect it from the weather.

In many cases carrier current apparatus is located at a considerabledistance from the power transmission line, making it necessary to use asignal transmission line, such as a coaxialcable, to transmit signalsbetween the carrier current apparatus and the power transmission line.Where such a signal transmission line has been utilized, it has been thepractice in the past to provide tuning means and impedance matchingmeans both between the carrier current apparatus and the signaltransmission line and between the signal transmission line and the powertransmission line.

In such systems a coupling capacitor is used to insulate the signaltransmission line from the power transmission line, and an inductance isprovided in series with the coupling capacitor between these two linesto tunethe capacitor and power line to series resonance at the signalfrequency. There is also provided a transformer for matching thecharacteristic impedance of the signal transmission line to theimpedance of the coupling capacitor and power transmission line, so asto provide for the maximum transfer of signals therebetween. A similartuning means and transformer has been used between the signaltransmission line and the carrier current apparatus. These tuning andimpedance matching means have reduced attenuation to the low levels.

It is an object of my invention to provide in such systems requiring asignal transmission line improved and simplified connecting meansbetween the carrier current apparatus and the power transmission line towhich it is connected. It is a further object of my invention to providesuch improved connecting means which requires 'a' minimum of adjustmentand maintenance except at the'carrier current apparatus.

It is another object of my invention to provide such improved andsimplified connecting means including a signal transmission cablebetween carrier current apparatus and the power transmission line, whichmeans requires a minimum of space anda minimum investment and which isrugged and reliable in operation.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, both asto its organization and manner of operation, together with furtherobjects and advantages thereof may best be understood by reference tothe following description taken in connection with the accompanyingdrawings in which Figs. 1, 2, 3 and 4 illustrate four modifications ofmy invention and Figs. 5, 6, 7 and 8 are graphical representations ofthe various characteristics thereof.

veniently be the center conductor of a coaxial transmission line, ofwhich the outer conductor is grounded. At that end of the transmissionline ll adjacent the power transmission line iii, the line II isconnected through one winding M of a transformer 55 to ground. Oneterminal of the other winding 56 of the transformerifi is grounded, andthe other terminal is connected through a suitable coupling capacitor llto the conductor I2 of the power transmissionline it.

An adjustable inductance i8 is connected between the carrier currentapparatus it and the signal transmission line H, and is so adjusted asto be series resonant with thecapacitive im-= pedanceof the systemincluding the signal transmission line H, the transformer it, thecoupling capacitor ll andtthe power transmission line it.

The windings l4 and it of the transformer 5 have a turn ratio equal tothe square root of the ratio of the impedance of the power transmissionline H as measured through the condenser H to the characteristicimpedance of the signal transmission line. That is, the square of thenumber of turns in the winding l6 corresponds to the impedance of thepower transmission line is, as measured through condenser ii, in thesame way as the square or the number of turns of the winding itcorresponds to the characteristic impedance of the signal transmissionline.

' Such a relation between impedances and turn ratio of the transformeri5 is desirable because attenuation in a transmission line .is minimumwhen it is connected at its ends to devices whose impedance, includingboth resistance and reactance, is equal to the characteristic impedanceof the transmission line. While it is true that such attenuation may befurther minimized by making the impedance of the connected devicespurely resistive, it has been found that the attenuation issatisfactorily low even though reactance be present, if impedances arematched, as stated above. The characteristic impedance of thetransmission line need not be matched exactly, as the attenuation doesnot increase rapidly as the connected impedances vary from a value equalto the characteristic impedance of the line.

In a typical example, where a coaxial cable of -70 'ohm characteristicimpedance is selected as accuses at oneirequency, and a-receiveroperative at a difierent frequency. The transmitter in the apparatus itis connected to the adjustable inductthe transmission line, the carriercurrent appara'tus operates at 100 kilocycles and the coupling condenserH has a capacity oi 0.002 microfarad, the impedance of the-power line ismay be assumed to be 400 ohms of pure resistance, and the impedance towhich the transmission line must then be matched is 882 ohms. Theimpedance ratio is then 12.6 and-the turn ratio of transformer 65 is3.4.

it has been determined that the elements shown in Fig. .1, adjusted asdescribed above, may be utilized for the eficient transmission ofsignals between the carrier current apparatus iii and the powertransmission line it with reasonably small attenuation through thesignaltransmission line H over substantial distances. The actual relationbetween attenuation through the signal transmission line i i and thelength oi that line is set forth hereinafter for certain situ= ationsencountered in practice.

In Fig. 2 the carrier current apparatus ill is connected through theinductance iii, and the transmission line it directly to one terminal ofthe coupling condenser ll, whose other terminal is connected to theconductor it of the power transmission line it. The inductance it is inthis case adjusted so that the whole system including the inductance it,the transmission line H, condenser ii, and the'power transmission lineit is resonant at the signal frequency. When this arrangement ofelements is used, omitting the impedance matching transformer is oi Fig.1, there is somewhat more signal refiection' and consequently greaterattenuation through the signal transmission line ii. It has beeniound;however, that even such greater attenuation is reasonably smallover some distances, and, in fact, not lntolerably large. The

relation between attenuation and the length of the signal transmissionline it for this arrangement or elements inFig. 2 is also set forthhereinafter for certain situations encountered in practice.

The apparatus illustrated in Figs. 3 and a are modlcations oi thearrangement illustrated in Fig. i. In Fig. 3 the carrier currentapparatus it is arranged for operationat two different frequencies. Forexample, the carrier current apparatus it may include a transmitteroperative ance i3 and transmission line H through two paths, one ofwhich includes an inductance is and condenser 2b in series, and theother of Which includes an inductance 2i and condenser 22 in series. Thereceiver in the apparatus it is connected through a conductor 23 to apoint between the condensers 2t and 22 and the inductance it.

The arrangement of such a transmitter and receiver together with meansfor coupling them to the power transmission line it includinginductances i3, iQand 2i and condenser i1, 2% and 22 forms no part ofthe present invention, and is described and claimed in my applicationfor United States Letters Patent, Serial No. 357,- 274. filed on Sept.18, 1M0, for Coupling apparatus, and assigned to the same assignee asthe present application.

Briefly, the adjustment of these elements is as follows. 'lhe inductanceis is adjusted to resonate in series with the condenser 28 at thetransmitter frequency, and the inductance it is adjusted to resonatewith the condenser H and transmission line 53 are series resonant at thetransmitter frequency and present a low impedance to the transmission ofcarrier current signals from the apparatus it to the transmission linel3.

At the frequency'to be received, the two series paths, one including thetransmitter in the apparatus it, the inductance it and condenser 26, andthe other including the inductance it and condenser ll, exhibitinductive or capacitive reactance, depending on whether the receiverfrequency is higher or lower, respectively, than the transmitterfrequency. The overall reactance of the series path including theadjustable inductance 2i and condenser 22 is adjusted to such a a valuethat the total reactance of the group including the transmitter, theinductances ii and 2t and the condensers so and 32 is equal in value,and opposite in character, to the reactance of the remainder of the pathfrom the apparatus it to the power line i3, such path including the ad-=justable inductance it and condenser ii. The inductance it and condenserll therefore. form one branch of a series tuned circuit, the otherbranch being formed by the transmitter in the,

apparatus iii, the inductances iii and 2! and the condensers 2t and 22,so that the potential at a point between these two branches tends torise to a high value at the receiver frequency. The conductor 23 whichisconnected to such point therefore transmits such high potential to thereceiver in the apparatus it.

In other respects the apparatus illustrated in Fig. 3 performs in afashion similar to that illustrated in Fig. 1, and provides emcienttransfer of signals with small attenuation over substantial lengths ofsignal transmission line it between the apparatus it and the powertransmission line it. The data presented hereinafter corre lating suchattenuation and line lengths for Fig. l is equally applicable forFig. 3.

heretofore, the connection shown in Fig. 3 and described in the abovementioned application has been used only where the carrier currentapparatus has been so near the power line i 3 that a transmission linehas not been needed,

since only in such arrangements could the connection 23 be made to apoint between the coupling condenser IT and the adjacent tuninginductances and condensers I8, I9, 20, 2I and 22.

through transmission line II being satisfactorilysmall and theconnection 23 may easily be made, thereby resulting in more efllcientoperation of the receiver, as well as easier adjustment and maintenanceof the apparatusas a whole.

In Fig. 4 the transmission line II, transformer I5 and condenser II areutilized to transfer carrier, current signals between the powertransmission lines I3 and two carrier current apparatus, including theapparatus I0 and another apparatus 30. Each such apparatus may include acarrier current transmitter, and a receiver operating at the samefrequency, each apparatus as a whole being operative at a differentfrequency.

The apparatus I0 is connected to the adjustable inductance I8 andtransmission line II through a condenser 3| shunted by an adjustableinductance 32. The apparatus 30 is connected to a point between theinductance I8 and transmission line II through a path serially includingan adjustable inductance 33 and a shunt combination of a condenser 34and an adjustable in-. ductance 35.

The adjustment of the apparatus of Fig. 4 is as follows. The paralleltuned circuit including the condenser 3I and inductance 32 is madeparallel resonant at the frequency of the apparatus 33. Similarly, theparallel circuit including condenser 34 and inductance 35 is madeparallel resonant at the operating frequency of the apparatus Ill. Theinductance I8 is then adjusted so that the apparatus Iii, inductance 32,condenser 3I, inductance I8, transmission line II, transformer I5,coupling condenser I1, and the power line I3 are resonant at theoperating frequency of the apparatus Ill. At this frequency the parallelresonant circuit including condenser 3d and inductance 35 ofiers a highimpedance so that signals transferred between apparatus Iii and thepower line I3 are not dissipated in the apparatus 30.

In similar fashion, the inductance 33 is adso that the groundedterminals of the carrier current apparatus may in each case beconnected, instead of to ground, to a second conductor of the powertransmission line I3.

In many situations it is entirely satisfactory to ground one terminal ofa carrier current apparatus, where the attenuation caused by suchconnection is not unduly great. By connecting the two terminals of thecarrier current apparatus to two different conductors of the power lineI3, signals may be transmitted through the power line I3'with somewhatless attenuation, so that longer transmission distances may be achieved.

, In the case of the apparatus illustrated in Figs. 1, 3 and 4, it isnot necessary to duplicate all the equipment for connecting the carriercurrent apparatus to the power line I3, since in each case,

the two terminals of the secondary I6 of transformer I5 may be connectedthrough separate coupling condensers to two different conductors of thepower line I3. In other respects the apparatus for interphasetransmission may be exactly the sameas that illustrated in Figs. 1, 3

justed so that the apparatus 30, the inductance 35, condenser 34, theinductance 33, the transmission line II, transformer I5, couplingcondenser I1, and power line I3, are all resonant at the frequency ofoperation of the apparatus 30. At this frequency the circuit includingcondenser 3I and inductance 32 offers a high impedance so that signalstransferred between the power line I3 and apparatus 30 are notdissipated in the apparatus I0. Insofar as the present invention isconcerned, the apparatus I0 and 30 respectively cooperate with theinductances I8 and 33, and with transmission line II, transformer I5,coupling condenser I1 and the power transmission line I3 in the same-wayas corresponding elements .in the apparatus of Fig. 1. The relationbetween attenuation and signal transmission line length is the same asthat set forth hereinafter for Fig. 1.

While the apparatus in Figs. 1 through 4 has been illustrated asimpressing a signal between a single conductor I2 of the power line I3and ground, it is within the scope of my invention to and 4.

One of the conditions termining how a simplified arrangement accordingto my invention may be utilized is the capacity of the couplingcondenser II. This capacity to a substantial extent determines thereactance presented to the end of the transmission line II adjacent thepower line I3, and hence is important in considering how muchattenuation may be produced through the transmission line II. In allcases hereinafter considered, it is assumed that the impedance betweenone conductor I2 of the power line I3 and ground is 400 ohms, whichvalue is substantially correct for any,

transmission line. Although power'transmission lines may have a somewhathigher impedance, the impedance is primarily resistive and consequentlyhigher impedance lines reduce attenua tion losses inthe transmissionline I I and make the resulting operation more efficient.

As a practical matter, the capacity of the coupling condenser I! isdetermined primarily by the amount of insulation it must afford betweenthe conductor I2 and ground. That is, the higher the voltage of thepower line I3, the more insulation there must be between the capacityelementsof the condenser II, and consequently the less the capacity ofthe condenser I1. At the present time certain sizes of condensers areavailable for.

use as coupling condensers on power lines of particular voltages, asindicated in the following tabulation:

' Number Capacity Line voltage. kv. condensers for conden- 2?? in seriesser, pf. p y

I In Fig. 5 values of attenuation between the carrier current apparatusI0 and the power line I3 are plotted as ordinates against correspond-''ing lengths of the transmission line II, plotted as abscissae, forseveral conditions of operation of the apparatus of Figs. 1 through 4.All the to be considered in de-- I l. Accordingly, a

trated by the curves ti, the attenuation which is realized when neitherthe transformer to nor tuning apparatus adjacent the condenser it isutilized is reasonably small even though transmission line it is as muchas 1000 it. long. The attenuation realized when the transformer i5 isutilized, as indicated by the curve dd, is even smaller and is wellwithin usable limits for cable lengths greater than 2000 ft, even thoughno tuning device is used adjacent the condenser ii.

For purposes of the general discussion herein,

it has been assumed that there should be no more than two decibelsattenuation between the carrier current apparatus i and the power lineit for satisfactory operation. Certain operating considerations-maychange this value to some extent in certain particular situations,Carrier current apparatus, as generally utilized at the present time, iscapable of producing about 50 decibels gain in a received signal whenthe automatic volume control voltage is smallest. The total attenuationbetween carrier current transmitter and receiver must, therefore, neverexceed this value, even under the worst conditions of weather and thelike.

Accordingly, it is common practice to assume that an attenuations of 25to 35 decibels on. a

clear dry day can be tolerated between a carrier.

10 and the power line it is in the order of one or two decibels.

If, however, the distance along the power line it! between the pointswhere carrier current equipments are connected thereto are shorter thansuch maximum distance, more attenuation can be tolerated tl rough thetransmission line determine the maximum us able length of thetransmission line H in any particular case, in view of the aboveconsiders.-

utilized where the coupling condenser H has a capacity of .004microfarads. J Figs. 6, '1 and 8 are identical with Fig. excep that theyillustrate theattenuatio'n for various 1 lengths of the transmissionline I I when the couthrwehil may be utilized to determine. the maxi-.

mum usable length of the transmission line H. It the transmission line Hwhich is to be used approaches a. length equal to one quarter wavelength of the carrier current wave at the operating frequency,additional consideration must be accuses given to the possibility oftuning the apparatus by means of the inductances l8 and If thetransmission line it, in any particular case, is made slightly greaterthan one quarter wave length long, the capacityof the power line i3 andcondenser ii appearsat the carrier current apparatus iii as inductance,since a quarter wave length line acts as an impedance inversion transformer. In such acase the inductance Hi cannot be used to tune theequipment, and if it be desired to use such a transmission line, it isnecessary to replace the inductance it with a condenser or with acapacitive combination of a condenser and an inductance. In any case,when the equipment is properly tuned by the inductance id, or by anysuitable means, th attenuation is as illustrated in the curves of Figs.5 through 8, even though the length of transmission line it be exactlyone quarter wave length, or a multiple thereof.

One form of the transmission line H which is especially suitableiorconnecting the carrier current apparatus 0 to the condenser H is acoaxial cable insulated with a rubber compound. The dielectric constantof one particular rubber compound used in practice is 3.3, so that aquarter wave length of the transmission line I i at 150 kilocycles is903 ft. at 85 kilocycles is 1,590 ft., and one quarter wave length at 50kilocycles is 2,710 ft. It should be remembered in making any particularinstallation that when the length of transmission line H approaches suchvalues an inductance may be incapable of tuning the equipment at thecarrier current apparatus i0.

If the exact operating frequency at which the system'is to operate isnot known when it is desired to determine how long the transmission lineH may be, the determination should be made on the basis of the lowestfrequency which may act ually be used, sincesuch' determinationgenerally indicates that the transmission line H is shorter than thatwhich would be determined for .a higher operating frequency. However,when considering whether the transmission line II I may approach a.quarter wave length or multiple thereof in length, it may be necessaryto determine the'actual operating frequency.

As illustrated by the curves of Figs. 5 through 8, it has been mind thata. transmission line H may be utilized to connect a carrier currentapparatus Hi to a. power line IS without the necessity of providingtuning means at the connection between thetransmission line and thepower line, and also that an impedance matching transformenmay beomitted. It is within the scope of tions, the curves 40 and 4| of Fig. 5may be my invention .to utilize such a transmission line with tuningmeans only at the' carrier current apparatus ID in any type of carriercurrent system having a signal transmission line, of which 7 severalforms have been illustrated.

While I have shown anddescribed a particular embodiment of myinventiomit .will be obvious to aim in the appended claims to cover allsuch changes and modifications as fall within the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent in the UnitedStates is:

1. An arrangementfor coupling carrier current apparatus to a power linespaced a substantial distance therefrom comprising, means including atransmission line extending between One quarter wave length acceptspower line from said transmission line and matching the impedance ofsaid transmission line to the impedance of said condenser and powerline, and means for transmitting signals between said carrier currentapparatus and said transmission line, the impedance of said power linemeasured through said condenser, transformer, and transmission linebeing reactive, said last mentioned means having a reactance equal andopposite in character to the reactive component of the impedance of saidpower line measured through said condenser and transmission line,whereby attenuation of carrier current signals transmitted between saidpower line and said-apparatus is minimized.

2. An arrangement for coupling carrier current apparatus to a power linespaced a substantialdistance therefrom comprising, means fortransmitting signals between said apparatus and line comprising 'atransmission line, a coupling condenser and transformer connectedbetween' said transmission line and said power line, the impedance ofsaid power line measured through said condenser and transformer beingreactive 1 whereby substantial attenuation is produced in plingcondenser and transformer connected between said transmission'line andsaid power line, the impedance of said power line measured through saidcondenser being reactive, said transformer having an impedance ratioeflective to match said impedance with the characteristic impedance ofsaid transmission line, substantial attenuation being produced insignals transmitted through said transmission line in spite of saidtransformer because of the reactivecomponent of said impedance, andmeans coupling said apparatus to said transmission line for tuning theimpedance of said power line and coupling condenser through saidtransmission line and transformer, whereby said attenuation is reduced.

4. An arrangement for coupling carrier current apparatus including atransmitter and a receiver operative at different frequencies to a powerline spaced a substantial distance therefrom comprising, a transmissionline, a coupling condenser and transformer connected between said lines,said transformer having a turn ratio effective to match the impedance ofsaid transmission line to the impedance of said-power line andcondenser, the impedance of said power line measured through saidcondenser, transformer, and transmissionline being reactive at both thefrequencies of said transmitter and receiver, means for connecting saidtransmitter to said transmission line, said connecting means havingv areactance at the frequency of said transmitter equal and opposite incharacter to the reactance of said power line measured through saidcondenser, transformer, and transmission line, and

means for connecting said receiver in shunt to i said transmitter andatv least a portion of said connecting means and for adjusting thereactfrequency of said receiver to a value equal and opposite incharacter to the reactance of the remainder of said connecting means,said transmission line, said coupling condenser, said transformer, andsaid power line at the frequency of a said receiver.

5. An arrangement for coupling-a first and a second carrier currentapparatus, having respectween said lines, the turn ratio of saidtransformer being suitable to match the impedance of said transmissionline to the impedance of said power line and condenser, the impedance ofsaid power line measured through said condensers;

transformer, and transmission line having a reactive component at bothsaid frequencies, means for connecting said first apparatus to saidtransmission line, said connecting means havmg a reactance at said firstfrequency qual and opp site in character to said reactive component andhaving high impedance at said second frequency,

and means for-connecting said second apparatus to said transmission linehavingva reacts-nee at said second frequency equal and oppodte incharacter to said reactive component and having high impedance at saidfirst frequency.

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